1. Field of the Invention
This invention relates to injection molding screws and more particularly to injection molding screws for use in processing heat sensitive polymeric materials. The invention also relates to an improved process for plasticating such materials in an injection molding machine.
2. Description of the Prior Art
The injection molding process involves feeding a particulate thermoplastic polymer into the hopper of an injection molding machine and thence through a zone where the polymer is plasticated into a melt which is then forced into a mold that conforms the polymer to a desired shape.
The injection molding process is extensively used in producing thermoplastic articles of a wide variety of shapes and sizes. The success of injection molding is attributable to several aspects of the process, the most important being the achievement of high production speeds and minimal waste. The production speeds are maximized by the continuous aspects of the process while material waste is minimized by an ability to reprocess waste thermoplastic polymer.
In addition to the inherent advantages of the injection molding process itself, the types of articles susceptible of fabrication by injection molding have continually expanded through the development of thermoplastic polymers having particularly desirable properties for specific applications and the discovery of techniques for processing such polymers in injection molding apparatus. The result has been the availability of a wide range of injection molded products having desirable chemical and physical properties specified by consumers of such products.
Thus, while any thermoplastic polymer may be processed in injection molding apparatus, the ability to achieve a useful end product depends upon the chemical and physical properties of the polymer and how these properties are affected by the process conditions within the injection molding apparatus. In accordance with the need to adjust processing parameters in the injection molding operation, four main types of systems have been developed to melt and inject the thermoplastic polymer. These systems are referred to as (1) reciprocating screw, (2) single stage plunger, (3) two-stage plunger and (4) fixed screw ram. Combinations of these systems have also been used.
The most widely used system is the reciprocating screw. In this system, a supply of the particulate thermoplastic polymer is maintained in a hopper which feeds the inlet end of a plasticating screw encased in a constant diameter barrel. Typically, the screw has three separate sections: (1) a feed section, (2) a transition section, and (3) a metering section. The hopper transfers (usually by gravity) particulate thermoplastic polymer to the feed section of the screw which has a constant root and a single helical flight of constant pitch. The distance from the land of the screw flight to the root (i.e. the channel depth) is sufficient to permit conveyance of the particulate polymer along the helical channel, while also providing for the application of shearing force to the polymer. The mechanical energy imparted to the polymer through the application of shear is converted to thermal energy and thereby the temperature of the polymer begins to rise. The screw conveys the polymer to the transition section where the root constantly increases in diameter with a resultant intensification of the shear and compression forces to which the polymer is subjected. In the transition section, the polymer begins to melt and be mixed, the objective being the eventual formation of a homogeneous, melted mass of polymeric material. From the transition section, the polymer proceeds to the metering section where compression and average shear are at their maximum levels and where the mass flow rate of polymer along the plasticating screw is regulated.
From the metering section, the polymeric melt is deposited in front of the discharge end of the screw. A backflow valve, which is mounted on the discharge end of the screw, prevents the reverse movement of material onto the screw. As the material is so deposited, the screw retracts until a predetermined space beyond the discharge end of the screw is filled with the polymer melt. When the desired amount of polymer accumulates in that space, the screw becomes a plunger, is moved forward, and forces the polymer melt into a mold or molds of specific shape. The melt is allowed to cool within the mold before the pressure is released. After cooling, the article thus formed is ejected from the mold and the process is repeated.
Although the three section screw is by far the most popular for use in the injection molding process, other screw designs have been used. For example, an injection molding screw may have more than one metering section. But common to all injection molding screws is the progressive average increase of root diameter from the inlet end to the discharge end with the feed section of the screw being characterized by low average shear and low compression and the discharge end being characterized by high average shear and high compression. When used herein, the term "high shear" means the maximum average mechanical force applied to the polymeric material when the material is subjected to high compression; and the term "high compression" means the maximum pressure to which the polymeric material is subjected during the plastication process.
Although conventional injection molding screws are satisfactory for the processing of many polymers, heat sensitive polymers present unique problems and have caused workers in the art to seek alternate screw designs. As used herein "heat sensitive polymers" means those thermoplastic polymers having threshold decomposition temperatures close to, or in the range of, the temperatures necessary for plastication. The difficulty in processing heat sensitive polymers with conventional injection molding screws centers around the inability of such screws to provide the necessary homogeneity in the final polymer melt without exceeding the time/temperature limits for preventing thermal decomposition of the polymer. Examples of such heat sensitive polymers are the chloro- and fluoro-hydrocarbon polymers and the polyesters.
In applications, for example, where the injection molded article is used to contain food, beverages or the like, even minute amounts of a polymer's thermal decomposition products may adversely affect taste; and because the polyesters, particularly polyethyleneterephthalate, are useful in this application, the thermal decomposition products thereof, specifically acetaldehyde, must be eliminated by proper processing to prevent imparting undesirable taste to the food or beverage.
Thus, in accordance with the present invention, an injection molding screw is provided which substantially eliminates thermal polymeric decomposition of heat sensitive polymers in the injection molding process. Further, the present invention provides a process for plasticating such polymers in an injection molding machine while avoiding thermal polymeric decomposition.